The present invention relates to an improved cell hood and more particularly an improved hood for use on an aluminum reduction cell.
In the electrolyte process for the production of aluminum a reduction cell is employed. The reduction cells presently in commercial use vary greatly in both types and size. Most cells in use fall into one of the major categories of Horizontal Stud Soderburg and Vertical Stud Soderburg, which employ self-baking anodes, and Centerwork Prebake and Side Worked Prebake which employ prebaked anodes. Sizes ranges from about 45,000 amperes to 225,000 amperes. The typical cell comprises a hollow pan of carbon which is supported on a bed of insulating material. The carbon pan is provided with embedded steel rods to which current is fed such that the pan acts as a cathode in the reduction cell. The hollow pan is charged with a molten aluminum bath and a charge of the electrolyte comprising molten cryolite (Na.sub.3 AlF.sub.6) and alumina (Al.sub.2 O.sub.3) floats on the aluminum layer. One or more carbon anodes are immersed into the molten charge. With the passage of current to the cathode the Al.sub.2 O.sub.3 is dissolved in the cryolite and is disassociated. In the course of the electrolyte process, various gases such as carbon monoxide and carbon dioxide and particulate matter such as metallic sodium are generated in the reduction cell. In this age of environmental awareness, various developments have been made in the collection and treatment of the aforenoted emissions and treatment systems have been developed in recent years which are highly efficient, as for example, the apparatus disclosed in U.S. Pat. No. 3,470,075 to Johnson.
Currently, the greatest problem presently encountered for the further reduction and elimination of the emissions generated in the electrolyte process lies in the area of designing an efficient collection hood to fit the reduction cell. Known collection hoods, particularly collection hoods for a centerbreak cell, suffer from the disadvantage that they do not seal sufficiently tightly to prevent the escape of a significant percentage of environmentally damaging emissions. Thus, even though emissions extracted from within the hood can be conveyed to highly efficient treatment plants, a significant percentage of emissions leak from the hood and are never treated.
A typical hooding arrangement is disclosed in U.S. Pat. No. 3,948,749. As set forth in said U.S. Pat. No. 3,948,749 traditionally the hood comprised a plurality of removable shields the size and weight of which allowed for convenient and easily removed by hand so as to allow easy access to the cell for inspection and servicing. This requirement for lightweight hand removable shields results in a need for a large number of shields in order to enclose the reduction cell thereby resulting in a corresponding increase in the number of sealing surfaces and sealing joints. In addition, in order to mitigate the effect of high magnetic fields which would occur with steel shields, the magnetic fields increasing the force necessary to remove the shields, and to reduce the weight of the shields, the shields are typically constructed of lightweight aluminum or aluminum alloys. It has been found that the employment of aluminum shields is generally undesirable. The melting temperature of the aluminum shields is less than the operating temperatures of the reduction cell which frequently results in warping of the shields which results in a loss of hooding efficiency as well as a corresponding increase in maintenance costs.
Accordingly, it is the principal object of the present invention to provide an improved reduction cell hood characterized by high collection efficiency.
It is a particular object of the present invention to provide an improved reduction cell hood having a reduced number of joints to be sealed.
It is a further object of the present invention to provide an improved reduction cell hood which is easily movable by hand.
It is a still further object of the present invention to provide an improved reduction cell hood which is not damaged by the high temperatures employed in the cell.
Further objects and advantages will appear hereinbelow.